1. Field of the Invention
The present invention relates to machine tools, in particular, relates to a precision vertical lathe and a tool head for the precision vertical lathe for use in a precision working of Fresnel lens forming dies and the like, and also relates to a rotary table apparatus for use as a rotary working table in a precision machine tool such as the precision vertical lathe.
2. Description of the Related Art
A vertical lathe is well known as a machine tool to perform a turning process, for example. Such a vertical lathe has a rotary table arranged in horizontal direction, a saddle movably toward axis (movable in X axis) and mounted on a cross rail extending toward the diameter of the rotary table, a ram mounted in vertical direction (in Z axis direction) to the saddle, and a tool holder for holding tools fixed and located at the lower portion of the ram.
In the vertical lathe having the configuration described above, a work material is fixed on the rotary table, the turning process for the work material is performed by moving the ram in Z axis direction under the rotation of the rotary table. The X axis position of the tools is changed by moving the saddle in X axis direction so as to perform the turning process for the work material about the center of the rotation of the rotary table.
In the making of a Fresnel lens forming die by the turning process using the tools, a tilting angle or a lens surface given by the tilting angle of a convex shape in a concentric circle can be given by a tilting angle of a convex shape in a concentric circle of the work material to be turned by the tools.
Because the tilting angle of the lens surface (Fresnel surface) in the Fresnel lens is changed on the position in the radius direction of the lens, it is necessary to change the angle of the tool by the changeable tilting angle of the lens surface.
For this subject, the conventional vertical lathe having the above configuration cannot make Fresnel lens forming dies because the tools are fixed on the tool holder (or a tool box) and this tool holder is fixed and mounted at the lower portion of the ram and the angle of the tools cannot be changed by a servo control and the like.
Further, in the conventional vertical lathe, a linear guiding portion is mounted in front of (at one surface in vertical direction) the cross rail and the saddle is mounted on the linear guiding portion. The saddle is thereby moved in X axis direction.
In the vertical lathe having the above configuration, the weight of the saddle and the ram mounted on the saddle acts, as a load deviation, on the front portion of the cross rail. This causes a torsional deformation in the cross rail. This torsional deformation of the cross rail causes the decreasing of the movement accuracy of the saddle in X axis and thereby the decreasing of the working accuracy. Thus, the conventional vertical lathe cannot perform a super precision turning process required to satisfy a high-degree precision for the Fresnel lens forming dies and the like.
A rotary table apparatus used in machine tools such as the vertical lathe described above has a base body as a fixed portion and a rotary table as a rotary portion. The rotary table is rotatably supported around the vertical axis on the base body by a radial roller bearing and a thrust roller bearing of a large diameter corresponding to the diameter of the rotary table.
The rotation driving or the rotary table is performed by engaging the ring gear of a large diameter fixed on the rotary table with a small gear of a small diameter in a driving side and by an electric motor for the rotary-driving of the rotary table through the gear train of this small gear.
In the conventional rotary table apparatus, the vibration caused by the electric motor and the gear train is propagated to the rotary table during the rotary driving of the rotary table by the electric motor. It is thereby impossible to achieve a high rotary accuracy. The relative assembling errors between the rotary table and a power transmission system prevents the transmission of the rotary power of the electric motor as a complete torque to the rotary table. The power deviation acts thereby on the rotary table. This causes the decreasing of the rotary accuracy of the rotary table.
In the conventional rotary table apparatus, because the rotary table is supported with a roller bearing, the rotary table vibrates during the rotation of the rotary table. This also decreases the rotary accuracy of the rotary table.
Because the rotary accuracy of the rotary table affects the working accuracy, it is impossible for the conventional rotary table apparatus used in the conventional machine tools to perform the super turning process with a high-working accuracy such as the turning process for Fresnel lens forming dies and the like.